"My sister has a lot of friends."
Translation:Moja siostra ma wiele koleżanek.
Normally, "wielu" should be used for masculine personal, and "wiele" for not masculine-personal.
I read a short answer from 2003 saying that in the context of people (so maybe friends as well) both are correct, but using 'wiele' seems old-fashioned; so probably it's better to just stick to the rule.
It's more complicated - or maybe it's just that English uses the word "colleague" a lot less often.
"Kolega" may be just some person from school/uni/work etc., you don't really need to have any kind of close relation with him to call him "kolega". Or you can actually like him, but just not include to the group you would call "przyjaciele". Not one of the closest friends. It's a very vague word, applicable in a lot of situations. English generally seems to use the word "friend" a lot more often, which can be seen just by the fact that in English you have Facebook "friends", but in Polish they're only "znajomi" (acquaintances).
Anyway, I made the word "colleagues" the default English sentence, but "friends" will still be a starred version as well.